In 1980, the Philips Company proposed a recording medium having a transparent substrate and a plurality of data pits, and accessed by irradiating a laser beam through the transparent substrate. The recording medium is referred to as a compact disk (CD) and becomes more and more prevailing ever since. Moreover, many types of CD, for example, CD, CD-G; CD-I, photo-CD, VCD, CD-R and CD-RW, have been suggested and realized. In 1995, due to the increasing demand for larger amount of information, a more advanced type of optical disk, digital versatile disk (DVD), is proposed to provide 4.7 GB data on a single sided disk with a diameter of 12 cm.
The density enhancement of DVD can be manifested from the following comparison. The CD family employs a laser beam having a wavelength of 780 nm and a lens with the N.A. (Numeral Aperture)=0.45 to access the data stored therein. The pitch between two adjacent tracks of the CD is about 1.6 .mu.m. On the other hand, the DVD family employs a laser beam having a wavelength of 650 nm and a lens with the N.A.=0.6 to access the data stored therein. The pitch between two adjacent tracks of the DVD is about 0.74 .mu.m.
Besides the pre-recording type optical media, multi-rewritable optical disks are also developed for the demand for the storage and modification of the information. Most of them employ phase-charge materials. As the development of various phase-change materials and the success of the direct writing technique, the multi-rewritable and erasable optical recording media are merchandised. For example, PD and CD-RW (650 MB) are produced in 1997, and 2.6 GB DVD RAM by DVD union, 3.0 GB DVD+RW disk by Philips.
As to the mass production of the pre-recording type optical disk, the data to be recorded in the optical disk are at first processed through scrambling, interleave and then encoded by EDC (error detect code) and ECC (error correction code) coding. Then the modulation and power of a laser beam is controlled through the encoded data to expose the photoresist film coated on a glass substrate. Next, the photoresist film is developed to form a patterned photoresist film. A series of pits arranged in a special form is copied by using the evaporating and plating method so as to produce a stamper for being to store data. By using the stamper, the substrate with pits can be mass produced by mold ejection. Afterward, the substrate is coated with dielectric layer and reflection layer by sputtering. Finally, a protective layer is applied thereon by spin coating. Therefore, the pre-recording type optical recording medium is completed. To form the smaller pits or the pits with a smaller pitch, the wavelength of the laser beam can be reduced by using the blue light or UV light. Alternatively, the N.A. of the lens system can be further increased so as to increase the data density for the same size of a disk.
Besides, the recording density can be increased by using a more efficient encoding scheme, reducing the size of the pit and track pitch, or using multilayer techniques.
However, there are still several considerable problems present in the above approaches. The efficiency of the encoding scheme is limited by the requirement of the error detection and correction code. The scale down of the pit and track pitch is limited by the resolution of the optical instrument. The employment of a shorter wavelength is influenced by the optical response of the material used, and the stability and cost of a semiconductor laser. The use of a larger N.A. value is inherently limited according to the optics theory and the manufacturing ability. The use of multi-layer techniques is effected by the reading and recording reliability. Finally, the optical diffraction limit plays essential role in the above-mentioned limitation of the size of the pit and track pitch.
Recently, the optical near-field technique becomes more attractive to the researcher involved the development of the optical recording medium. For example, it is reported that the pit pitch of 40 nm.about.8 nm can be realized by the near-field probe optical recording. The pit pitch can be further reduced to 35 nm by using super-resolution N.A. with a solid immersion lens. However, in the above technique, there is a practical problem for the design of the optical probe due to the requirement of controlling the distance between the probe and the reading surface of the optical medium through the shear force feedback control of the probe. Moreover, the probe system for this near-field technique is not compatible with the present system, and it is possible to damage and break the probe.
Moreover, a super-resolution structure is proposed wherein the near-field effect is realized by the special design of the multi-layer based on the nonlinear optics mechanism other than by the probe.
However, there are some disadvantages present in the above method. As shown in FIG. 1, the accessing to the high density optical recording medium is conducted by the near-field probe. In this method, it is necessary to maintain a constant distance about 100 nm between the probe and the surface of the disk to achieve the effect of the optical near-field. Due to the maintenance of the constant distance, it is necessary to design a control system to control the dynamic shear force generated by the air flow between the probe and the surface of the disk. As a result, the design is more complicated and is not compatible with the present reading and writing system. Moreover, in order to control the distance, the surface of the disk is usually scratched or the probe is broken resulting in the malfunction of the reading and writing system.
For the super-resolution structure, as shown in FIG. 2, an optical nonlinear film having the thickness of 15 nm is incorporated into the disk structure thereby conducting the control of the optical near-field effect. The super-resolution structure is easy to implement because the optical nonlinear film may be formed during the manufacturing process of the disk. Furthermore, similar to the present technology about the optical disk player, it is unnecessary to maintain a minute distance between the optical accessing head and the surface of the disk. As a result, the object of the high density recording is easily achieved by using this technique. However, the super-resolution effect is not generated by the nonlinear optical property of the super-resolution structure and the optical nonlinear film is not made of a dielectric material. Therefore, the physical mechanism of the super-resolution effect should be clearly exploited such that other possible materials and structures to generate super-resolution effect are developed.